U.S. patent number 4,077,778 [Application Number 05/796,781] was granted by the patent office on 1978-03-07 for process for the catalytic gasification of coal.
This patent grant is currently assigned to Exxon Research & Engineering Co.. Invention is credited to Nicholas C. Nahas, Charles J. Vadovic.
United States Patent |
4,077,778 |
Nahas , et al. |
March 7, 1978 |
Process for the catalytic gasification of coal
Abstract
A process for the production of synthetic natural gas from a
carbon-alkali metal catalyst or alkali-metal impregnated
carbonaceous feed, particularly coal, by reaction of said feed with
water (steam) in the presence of a mixture of hydrogen and carbon
monoxide, in a series of staged fluidized bed gasification reactors
(or gasification zones). The first reactor, or main reactor, of the
series is operated as an entrainment reactor and entrained solids
are carried over to the second reactor, or reactors, of the series.
A carbonaceous feed, or coal, is thus partially gasified in a
fluidized bed in the main reactor to form a product gas and a char,
and char is entrained within the effluent gases, separated
therefrom and then fed into the secondary gasification reactor, or
reactors, of the series, the entrained char constituting feed to a
secondary reactor, or reactors. Some char is also passed via dense
phase transfer from the main reactor to the secondary reactor to
maintain catalyst balance. More effective ultilization of the feed
carbon is possible by use of the reactor system operated in such a
manner than is possible by the sole use of the main reactor, even
when the latter is operated at the same or at more optimum
conversion conditions.
Inventors: |
Nahas; Nicholas C. (Morris
Plains, NJ), Vadovic; Charles J. (Houston, TX) |
Assignee: |
Exxon Research & Engineering
Co. (Linden, NJ)
|
Family
ID: |
24474666 |
Appl.
No.: |
05/796,781 |
Filed: |
May 13, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
617698 |
Sep 29, 1975 |
|
|
|
|
Current U.S.
Class: |
48/202 |
Current CPC
Class: |
C10J
3/482 (20130101); C10J 3/54 (20130101); C10J
3/721 (20130101); C10J 2300/0903 (20130101); C10J
2300/093 (20130101); C10J 2300/094 (20130101); C10J
2300/0973 (20130101); C10J 2300/0986 (20130101); C10J
2300/1823 (20130101); C10J 2300/1853 (20130101); Y02P
20/52 (20151101) |
Current International
Class: |
C10J
3/54 (20060101); C10J 3/46 (20060101); C10J
003/54 () |
Field of
Search: |
;48/197R,202,206,210 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lindsay, Jr.; Robert L.
Assistant Examiner: Kratz; Peter F.
Attorney, Agent or Firm: Proctor; L. A.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a Continuation-In-Part of copending application
Ser. No. 617,698 which was filed Sept. 29, 1975 in the name of the
same inventors as are named herein and now abandoned.
Claims
What is claimed is:
1. In a process for the production of synthetic natural gas by the
conversion of a solid carbonaceous feed, in the presence of a
carbon-alkali catalyst, by contact of said feed in a gasification
zone containing a fluidized bed of char, with steam and a mixture
of hydrogen and carbon monoxide gases added to said zone to
suppress reactions which compete with the formation of equimolar
concentrations of methane and carbon dioxide from a stoichiometric
equivalent of carbon and water, the improvement comprising
continuously supplying to a main gasification zone a solid
carbonaceous feed, a carbon-alkali catalyst, steam and a mixture of
hydrogen and carbon monoxide gases, gasifying said solids
carbonaceous feed at temperatures ranging from about 1000.degree.
to about 1600.degree. F and pressures ranging from about 100 psia
to about 1500 psia by injecting gases upwardly at superficial
linear velocities ranging from about 0.5 to about 3 ft/sec.
sufficient to devolatilize and react with the solid carbonaceous
feed to form char and gaseous effluent, maintain the char in
fluidized state and entrain within the gaseous effluent from about
5 to about 30 percent of the fine solid char, based on the weight
of total feed to said main gasification zone,
separating the entrained char from the gaseous effluent, and
feeding said char into a secondary gasification zone, recovering
from the gaseous effluent a synthetic natural gas consisting
essentially of methane and a synthesis gas comprising hydrogen and
carbon monoxide,
removing from said fluidized bed of char of said main gasification
zone from about 5 to about 30 percent additional char, based on the
weight of total feed,
feeding said char via dense phase transfer as an admixture of char
and carbon-alkali catalyst into said secondary gasification
zone,
continuously supplying to said secondary gasification zone steam
and a mixture of hydrogen and carbon monoxide gases, gasifying said
char at temperatures ranging from about 1000.degree. to about
1600.degree. F and pressures ranging from about 100 psia to about
1500 psia by injecting gases upwardly at superficial linear
velocities below those employed in said main gasification zone and
ranging from about 0.05 to about 1 ft/sec. sufficient to maintain
the char in fluidized state, but insufficient to entrain any
significant amount of the char,
withdrawing a gaseous effluent from said secondary gasification
zone and recovering synthetic natural gas consisting essentially of
methane and a synthesis gas comprising hydrogen and carbon
monoxide, from said gaseous effluent, recycling the recovered
synthesis gases as the hydrogen and carbon monoxide gases supplied
to the main and secondary gasification zones,
whereby more efficient utilization of the feed carbon is achieved
than can be obtained by the sole use of the main reaction zone,
even when said main reaction zone is operated at the same or
conditions more favorable for gasification of the feed carbon.
2. The process of claim 1 wherein the solid carbonaceous feed fed
into the main gasification zone is coal.
3. The process of claim 2 wherein the solid carbonaceous feed fed
into the gasification zone is coal, and said coal is impregnated
with potassium to produce said carbon-alkali catalyst.
4. The process of claim 1 wherein the mixture of hydrogen and
carbon monoxide gases are recycled to the main and secondary
gasification zones approximates a 3H.sub.2 :1CO molar
composition.
5. The process of claim 1 wherein the temperature of the main
gasification zone ranges from about 1200.degree. to about
1400.degree. F, and the pressure ranges from about 500 to about
1000 psia.
6. The process of claim 1 wherein the superficial linear velocities
of the gases entering the main reaction zone ranges from about 1 to
about 2 feet per second sufficient to entrain from about 10 percent
to about 20 percent of the fine solids char, the entrained char
being fed into the secondary gasification zone with from about 5 to
about 15 percent additional char which is fed via dense phase
transfer into the secondary gasification zone.
7. The process of claim 1 wherein the temperature of the secondary
gasification zone ranges from about 1200.degree. to about
1400.degree. F and the pressure ranges from about 500 psia to about
1000 psia.
8. The process of claim 1 wherein the superficial linear velocities
of the entering gases to the secondary gasification zone range from
about 0.1 to about 0.5 feet per second.
9. The process of claim 1 wherein the residence time of the
unreacted solids in both the main gasification zone and secondary
gasification zone ranges from about 3 to about 15 hours.
10. The process of claim 1 wherein the average particle size of the
char entrained within the gases fed into the secondary gasification
zone ranges below about 200 mesh.
11. The process of claim 1 wherein the average particle size ranges
below about 325 mesh.
Description
BACKGROUND OF THE INVENTION
Fuel oil and natural gas shortages have sparked renewed world-wide
interest in the development of processes that can produce clean
synthetic natural gas, or gas or pipeline quality, from
carbonaceous solids, particularly coal. Various processes, both
thermal and catalytic, are known for the gasification of coal to
produce pipeline quality gas. In gasification processes of these
types, raw gas mixtures are produced which include hydrogen, carbon
monoxide and methane. Since methane, the desired high BTU
component, cannot normally be directly produced within the gaseous
product in adequate concentrations to provide a pipeline quality
gas, the raw gases are upgraded in separate downstream reactors,
and processing units to produce additional methane.
A raw gas, after separation of the methane component, is
conventionally upgraded downstream of the main reactor in a series
of shift-methanation reactions which increases the methane content
of the gas. In a shift reaction, additional hydrogen is first
generated by reacting carbon monoxide and steam to produce carbon
dioxide and hydrogen, as characterized by the equation: CO +
H.sub.2 O.fwdarw. CO.sub.2 + H.sub.2. After removal of acid
components (e.g., CO.sub.2), the hydrogen is then reacted with
carbon monoxide in a methanation reaction to produce methane, as
characterized by the equation: CO + 3H.sub.2 .fwdarw.CH.sub.4 +
H.sub.2 O. There are, of course, many variables which determine the
efficiency of any given gasification process. The amount of methane
which can be directly produced in any given gasification process
vis-a-vis that which can be indirectly produced, however, is an
important variable in determining the efficiency of a coal
gasification process.
Although both thermal and catalytic coal gasification processes
have been generally known for many years, at least until recently,
neither type of process had proven of outstanding efficiency, one
type relative to the other. Each had its advantages and its
disadvantages. Various designs of each process type are thus being
widely investigated, and developed for possible commercial use. A
catalytic process of admirable merit is described in Application
Ser. No. 514,852, filed Oct. 15, 1974, by K. K. Koh et al. and not
abandoned. In this process, herewith incorporated by reference,
methane is produced in a catalytic gasification zone, suitably one
containing a fluidized bed, by reacting steam with carbonaceous
solids, particularly coal, in the presence of a carbon-alkali metal
catalyst, or an alkali-metal impregnated carbonaceous feed, and a
recycle stream of synthesis gas (H.sub.2 + CO). The catalytic
gasification reaction is conducted at temperatures ranging about
1000.degree. to 1600.degree. F, and at pressures ranging about 100
to 1500 pounds per square inch absolute (psia). A feature of this
process is that completing reactions are suppressed by the recycle
or synthesis gas, such that the net reaction products are
essentially methane and carbon dioxide, in accordance with the
equation: 2C + 2H.sub.2 O.fwdarw.CH.sub.4 + CO.sub.2. Product
methane, carbon dioxide, and the synthesis gas used as recycle to
suppress competing reactions are withdrawn from the gasifier,
passed through a heat recovery system, and then sent to a cryogenic
separation unit from which the methane and carbon dioxide are
separately recovered. In a typical coal, 1 mole of methane that is
recovered contains 98.7% of the energy of the 2 moles of carbon
gasified.
The Koh et al. catalytic process offers profound advantages over
prior art processes, both thermal and catalytic. Whereas the
thermal efficiency of thermal gasification processes, in
particular, is severely limited by the sequence of reactions, such
limitations can be avoided by the Koh et al. catalytic process. A
gas of high methane content can be directly produced. The
gasification can be conducted at high rate, even at relatively low
temperature. The reaction is substantial thermoneutral and,
although it is necessary to heat the reactants to reaction
temperature to initiate the reaction, essentially all of the heat
supplied to the reactor is recovered. Hence, there is very little
waste heat. The discovery that methane and synthesis gas can be
equilibrated in the reaction, i.e., maintained in equilibrium by
separation and recycle of synthesis gas to the reactor to produce
methane directly, has eliminated all need for downstream
shift-methanation reactions, and the thermal efficiency of this
process is significantly higher than that of prior art
processes.
Despite the relatively high efficiency, and other advantages
offered by this process, a further deficiency resides in the less
than total gasification of the feed carbon. A major source of
carbon loss is caused by backmixing of particles, and by the
elutriation of fines present in the feed coal, inclusive of the
elutriation of fines created by attrition within the gasifier. Not
only does the loss of the elutriated fines lower the amount of
carbon conversion that is possible, but this also results in high
catalyst losses, which effect is particularly manifest because a
considerably greater weight proportion of catalyst is contained in
the entrained fines than in the initial feed.
SUMMARY OF THE INVENTION
It is accordingly the primary objective of this invention to
obviate these and other deficiencies, particularly those associated
with catalytic coal gasification processes.
It is a specific object of this invention to provide a staged
catalytic coal gasification process, particularly a two-stage
gasification process, for the more effective gasification and
utilization of the carbon content of a carbonaceous feed.
Another object is to provide a further improved coal gasification
process of the type described in Application Ser. No. 514,852,
supra.
These and other objects are achieved in accordance with the present
invention, characterized as a process for the production of
synthetic natural gas from a carbon-alkali metal catalyst or
alkali-metal impregnated carbonaceous feed, particularly coal, by
reaction of said feed with water (steam) in the presence of a
mixture of hydrogen and carbon monoxide, in a series of staged
fluidized bed gasification zones, inclusive of a main gasification
zone, or zones, wherein the carbonaceous feed, or coal, is
partially gasified in a fluidized bed to form a product gas and a
char, and char is entrained within the effluent gases, separated
therefrom and then fed into a subsequent or secondary gasification
zone, or zones, as a major source of carbon feed to said secondary
gasification zone, or zones. Within the process more effective
utilization of the feed carbon, based on total weight of feed
carbon to the main gasification zone, or zones, is possible than by
the sole use of the main reaction zone, or zones, even when the
latter is operated at the same or at conditions more favorable for
gasification of the feed carbon.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE is a schematic flow diagram of a two stage gasification
process within the scope of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In a preferred embodiment, crushed or particulate coal impregnated
with an alkali-metal is fed into the fluidized bed gasification
zone of a main reactor of a series. Suitably, raw coal is crushed
to average particle sizes ranging from about 4 to about 325 mesh
(Tyler), preferably from about 8 to about 200 mesh, and then
impregnated as by spraying or otherwise admixing same with a
solution of an alkali-metal salt, or mixture of alkali-metal salts,
inclusive particularly of potassium salts, notably potassium
carbonate, and the coal then dried to form a suitable carbonaceous
feed.
The impregnated coal is fed into the fluidized bed gasification
zone of the main reactor, contacted, devolatilized and reacted
therein with water (steam) and a fresh or recycle mixture of
hydrogen and carbon monoxide, typically at a temperature providing
a H.sub.2 :CO molar ratio of about 3:1, at superficial linear
velocities ranging from about 0.5 to about 3 feet/second,
preferably from about 1 to 2 feet/second, sufficient to provide
from about 5 to about 30 percent, preferably from about 10 to about
20 percent, entrainment of the solid fines char, based on the
weight of total carbon feed to the fluid bed gasification zone of
the main reactor. In addition to the fine solids char which is
transferred by entrainment in the effluent gases from the fluidized
bed of the main gasification zone to the secondary gasification
zone, or zones, from about 5 to about 30 percent, and preferably
from about 5 to about 15 percent, based on the total weight of
carbon feed to the main gasification zone, is fed into the
secondary gasification zone, or zones, by direct dense phase flow.
Suitably, the depth of the fluidized bed of the main reactor ranges
from about 20 to about 140 feet, preferably from about 50 to about
120 feet, and the average particle size of the solid fines char
entrained in the effluent gases ranges below about 200 mesh,
preferably about 325 mesh.
The residence time of the unreacted solids within the main reactor
ranges generally from about 3 to about 15 hours, and preferably
from about 4 to about 10 hours. The temperature of the fluidized
bed reaction zone of the main reactor, or high entrainment reactor,
is maintained from about 1000.degree. to about 1600.degree. F,
preferably from about 1200.degree. to about 1400.degree. F,
essentially any pressure could be used but pressures ranging from
about 100 to about 1500 psia are most effective and pressures from
about 500 to 1000 psia are preferred. The fine solids char
entrained in the product gas stream from the gasification zone of
the main reactor is separated, preferably by one or a series of
cyclone separators, and fed into the fluidized bed of the secondary
gasification zone, or zones.
The gases entering the fluidizied bed of the secondary gasification
zone, or zones, is maintained at a velocity below that used in the
main gasification zone, or zones, and sufficiently low to avoid
significant entrainment of the fine solids char. Sufficiently long
residence time is required to permit substantially complete
conversion of the carbonaceous matter of the char to gases.
Suitably, the superficial linear gas velocities within the
secondary gasification zone, or zones, ranges from about 0.05 to
about 1.0 feet per second, preferably from about 0.1 to about 0.5
feet per second, this avoiding significant entrainment of the fine
solids char within the effluent gases. Generally, essentially
complete gasification of the fine solids char is obtained at
residence times ranging from about 3 hours to about 15 hours,
preferably from about 4 to about 10 hours.
The secondary gasification zone, or zones, can be operated at the
same or at different temperatures and pressures, either greater
than or less than those employed in the main gasification zone, the
temperature generally being selected to achieve an optimum balance
between the desired product gas composition and carbon gasification
rate. Suitably, the secondary gasificaton zone, or zones, is
operated at temperatures ranging from about 1000.degree. to about
1600.degree. F, and preferably at temperatures ranging from about
1200.degree. to about 1400.degree. F. Again, essentially any
pressure could be used but pressures ranging from about 100 psia to
about 1500 psia are most effective and pressures from about 500
psia to about 1000 psia are preferred, and during any given period
of operation generally approximates that employed in the main
gasification zone, or zones. In general, the depth of the fluidized
bed of the secondary gasification zone, or zones, approximates that
of the main gasification zone differing, in any given operation,
only by a small amount wherein a differential head is maintained to
transfer fine char solids from the main gasification zone. The
product gas can then be passed through a heat recovery system for
recovery of heat values. Product methane and carbon dioxide from
the gasification zones, after removal of the fine solids char, is
separated from the synthesis gas, or mixture of hydrogen and carbon
monoxide, as in a cryogenic separation, and the latter is recycled
to the main and secondary gasification zones to suppress reactions
which compete with the formation of methane and carbon dioxide.
These and other features of the present process will be better
understood by reference to the attached FIGURE, and to the
following description and examples which make reference to a
specific two-stage gasification process depicted in the FIGURE. The
FIGURE depicts, in schematic fashion, a preferred embodiment of
this invention.
Referring generally to the FIGURE, there is shown a pair of
gasifiers, a main gasifier 10 and a secondary gasifier 20, each of
which contains a fluidized bed 11, 21 of particulate coal solids.
Raw particulate coal, impregnated, e.g., with 10-30 percent
potassium carbonate, based on the total weight of the coal, or a
potassium-coal catalyst, is added with water (steam) into the
fluidized bed 11 of the main gasifier 10. The feed coal and
alkali-metal salt is fed pneumatically either to the bottom or side
of the main gasifier 10, with steam as the transfer medium as via
lines 13, 14. Fresh or recycled synthesis gas, approximating
3H.sub.2 :1 CO in molar composition, is added with the steam, the
velocity of the gases being adequate not only to fluidize the bed
11 but also to entrain from about 5 to 30 percent, preferably from
about 10 to 20 percent of the fine solid coal particles, based on
the weight of coal feed to the main gasifier 10, and sweep these
from the main gasification zone. Ash is optionally withdrawn from
the bottom of the main gasifier 10 as via line 12, and the level of
the fluidized beds 11, 21 within the gasifiers 10, 20 are
maintained by a direct dense phase flow line 19 which interconnects
the two gasifiers 10, 20. Some flow of solids from the main
gasifier 10 to the secondary gasifier 20 is necessary, but
essentially limited to that necessitated for the purge of ash from
the main gasifier 10 and that necessary to maintain gasifier 10 in
catalytic balance. The driving force for this flow is regulated by
the difference provided between the solids levels of beds 11, 21
within the two gasifiers 10, 20. Effluent gas from the main
gasifier 10 is removed from the overhead portion of the gasifier
and conveyed by an appropriate line 16, with effluent gas conveyed
via an appropriate line 26 from the secondary gasifier 20, to one
or a series of cyclone separators, e.g., a primary cyclone 30 and a
secondary cyclone 40. Gas from primary cyclone separator 30 is
removed via line 31 and passed to the secondary cyclone 40, which
gas exits therefrom and is fed via line 41 into a product gas
separation zone 50. A product gas consisting essentially of methane
is removed via line 51 and synthesis gas is removed via line 52.
Fine solids are returned to the secondary gasifier 20 from the
primary and secondary cyclone separators 30, 40 via lines 32, 42,
respectively. Synthesis gas is recycled via lines 52, 44 and 52, 45
to gasifiers 20, 10, repsectively, to suppress reactions which
compete with that favoring the formation of equimolar quantities of
methane and carbon dioxide from a stoichiometric amount of carbon
and water in accordance with the equation: 2C + 2H.sub.2
O.fwdarw.CH.sub.4 + CO.sub.2. Steam is fed via lines 24, 25, with
recycle synthesis gas, into the bottom of gasifier 20 as a reactant
to fluidize bed 21.
In accordance with this invention, it has been found feasible to
effectively utilize as high as about 95 to 99%, and higher, of the
feed carbon fed to the main gasification zone, or gasifier. This is
sharply contrasted with the operation of a single gasifier which,
at optimum conditions, provides only from about 70 to about 85
percent utilization of the feed carbon which can be gasified to
provide useful end products. The following comparative data are
illustrative of the advantages resultant from the practice of this
invention.
The data given in Table I below are illustrative of typical
catalyst levels and the carbon conversion levels obtained, for two
different cases, in the operation of the preferred two-stage
catalytic gasification process, employing 10 weight percent
potassium carbonate on Illinois #6 type coal, at the conditions
described.
TABLE I ______________________________________ Main Secondary Unit:
Gasifier Gasifier ______________________________________ Process
Conditions: Temperature, .degree. F 1400 1400 Pressure, psia 50 50
Gas Velocity, Ft. per second 2 0.5 Residence Time of Solids, Hrs. 5
7 ______________________________________ ESTIMATED CARBON &
CATALYST FLOWS TO GASIFIERS Total Carbon Ash Potassium Case 1 #/Hr.
______________________________________ (Carryover = 10% Entrain-
ment of Feed) Feed Coal to Main Gasifier 100 66.5 17 5 Carryover by
Entrainment to Secondary Gasifier 10 6.0 4.0 1.4 Withdrawal by
Dense Phase Transfer from Main Gasifier 14.7 4.4 10.3 3.6 Total
Feed to Secondary Gasifier 24.7 10.4 14.3 5 Ash Withdrawal from
Secondary Gasifier 15.0 0.7 14.3 5 Carbon Gasification Percentage
Main 84.4 Secondary 14.6 TOTAL 99.0
______________________________________ Total Carbon Ash Potassium
Case 2 #/Hr. ______________________________________ (Carryover =
20% Entrain- ment of Feed) Feed Coal to Main Gasifier 100 66.5 17 5
Carryover by Entrainment to Secondary Gasifier 20 12.0 8 2.8
Withdrawal by Dense Phase Transfer from Main Gasi- fier 9.0 2.7 6.3
2.2 Total Feed to Secondary Gasifier 29.0 14.7 14.3 5.0 Ash
Withdrawal from Secondary Gasifier 15.0 0.7 14.3 5.0 Carbon
Gasification Percentage Main 77.9 Secondary 21.1 TOTAL 99.0
______________________________________
From these data it will be observed that 99 percent of the feed
carbon, based on the weight of coal feed to the main gasifier, can
be gasified to useful end products. The secondary gasifier, which
is fed by fines carryover and some direct feed dense phase transfer
of char from the main gasifier, accounts for about 14 to 21 percent
of the total conversion of the carbon to useful end products
whereas from about 78 to 84 percent of the conversion occurs in the
main gasifier.
It is apparent that various modifications can be made in the
process without departing the spirit and scope of the present
invention. The present process provides generally, a high
entrainment main gasifier, or group of gasifiers, wherein after
separation of the fine solids char from the product gas, the char
is fed into a secondary gasifier, or group of gasifiers, as a
principle source of carbon feed. In contrast to the main gasifier,
or group of gasifiers, the entrained solids from the main gasifier,
or group of gasifiers, constitutes a major source of carbon feed to
the secondary gasifier, or group of gasifiers, although some carbon
feed is also transferred from the main gasifier, or gasifiers, by
dense phase transfer. The secondary gasifier, or gasifiers, are
necessarily operated at low gas velocities to avoid significant
fines entrainment, and long residence times are provided to effect
essentially complete conversion of the carbon-feed to useful
gaseous products. In a preferred mode of operation a plurality of
main gasifiers are employed to feed char to a single secondary
gasifier.
* * * * *